1. Field of the Invention
This invention relates to a yarn control system and apparatus for use in the manufacture of non-woven, non-knitted fabrics which have a pile supported by a prewoven or non-woven backing cloth. The invention is particularly suitable for use with yarn feed systems of cut-pile and loop-pile, straight, single sliding, staggered and double sliding needle bar tufting machines, especially carpet tufting machines.
2. Description of the Prior Art
Most pile fabrics are manufactured on tufting machines which incorporate needle bars extending transversely of the length of fabric to be formed, and the needle bars have a plurality of needles therein, and are caused to move up and down at high speed, so as to force the needles thereon through a pre-woven backing cloth which is advanced in the longitudinal direction of the fabric. A yarn from a supply is fed by a feed roller arrangement to each needle, and as soon as the needles have been inserted through the backing cloth, a looper/hook engages with the yarn, thus forming a loop of yarn when the needle is withdrawn again. This forms a loop pile, but if a cut pile is required, the loop is held by a looper and is subsequently cut.
In order to obtain a smooth surface to the pile material, each loop or cut length should be of the same height and the tufting machines are designed with this in mind. However, if the tension in the yarn from its supply to its needle varies, a level pile is unlikely to result, because any loop or cut length of yarn, formed from a yarn under high tension, once it is supported in the backing cloth, will tend to shrink and thus will eventually be too short. In contrast to this, the loop or cut length of yarn may be too long if there was low yarn tension when the needle inserted it in the backing cloth.
When it is desired to form a patterned fabric, different yarns are used, e.g. of different colours, and a pattern is achieved by moving the needle bar axially, or by moving the backing cloth transversely relative to the needle bar, so that the loops of pile being inserted into the advancing backing cloth are shifted transversely, resulting in a diagonal stitch on the back of the backing cloth. The distance between needles on a needle bar can be of the order of 1/20" to 3/8", and is known as gauge. It is possible to shift needle bars by one, two or perhaps more gauges and a two gauge movement is known as double gauge movement. This means that in such instances, more yarn will be required, because the pieces of yarn to be formed into such loops will still be attached to the backing cloth as a result of the previous loop (or cut length) having been formed. This is likely to mean that when such a loop is formed, for example, after a single gauge movement, it will be formed with the yarn under greater tension than a straight loop. Likewise, when a loop is formed after a double gauge movement of the needle bar, the yarn is likely to be under even greater tension.
The above described machines have yarn feed systems which cannot really cope with the requirement for different yarn lengths when the needle bars are shifted axially. They are set up so that the yarn feed is set somewhere between the smallest and the largest amount required. This means that on some stitches or loops, the yarn is fairly slack when it is engaged by the looper/hook, and on others it is very tight. This shows itself in the finished pile material as an uneven surface, because some rows of pile material, e.g. cut lengths or loops, are too long, and others are too short. To some extent, the problem can be reduced by shearing the surface of the pile. However, this does not fully solve the problem. Excessive shearing would result in considerable yarn wastage, but in practice the problem is still there because of the differential height between adjacent tufts results in high tufts overlying lower tufts during shearing, thus preventing constant presentation of the tufts to the shearing blades.
This means that pile materials, and especially tufted carpets, formed on existing tufting machines, and especially patterned pile material, end up with ugly lines (movement/shift/cam lines) across their surface from the uneven heights of the rows of pile.
Similar problems can also result because those rows of yarn which are inserted with the yarn under too much tension will cause the backing cloth to be deformed slightly, forming slight ridges transversely of the cloth. This again forms ridges and troughs in the finished surface of the carpet, and known as the cam movement or shift line effect.
Furthermore, when rows of yarn are inserted under tension, the weft of the backing cloth can be strained, and can reduce the dimensional stability of the backing cloth.
Lines across the surface, or at least spasmodic differences in pile height, can also result if different yarns are under different or varying tensions, for whatever reason. It will be readily apparent that even in plain pile materials, the tension under which the loop is formed can eventually result in cut or looped lengths of pile being of different heights.